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wks

Glazing Terracotta

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Hi All!

 

I am new to the Ceramic Arts Community! 

 

I made a few pieces both hand built and on the wheel with terracotta clay. I would like to glaze and fire my pieces in one fire with a clear matte finish. So my questions are...

 

1. Is it possible to skip the bisque fire and just glaze it and fire it once

2. What cone should I buy for my glaze?

3. Does anyone have any recommendations for clear matte glazes that would work for terracotta

 

Thanks in advance!

 

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Terra Cotta is such a rich color maybe you should think about constructing colors.check out some of the low fire glaze recipes on Ceramic Arts Daily in the upper left corner. 

Google Lisa Naples... she fires to her terra cotta clay's maturity at ^2. Her work is really dynamic. 

 

Marcia

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a clear matte is an oxymoron with glazes. can't have a clear that is also matte, the microcrystalline structures that make a glaze matte preclude it being clear. some clears are somewhat translucent, as long as it's not that way from just being a clear glaze that is underfired.

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I've used a Duncan Satin Glaze---cone 06 Translucent Overglaze that is pretty nice. SN351 I could see informally describing it as close to a clear matte, in terms of affect...tho translucent satin is correct. 

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If you are firing the terracotta to Cone 5 that is 1196C, and you are really pushing the upper limits of terracotta. At that temperature the clay is likely to be a very dark red becoming dark brown. Also you may have some bloating where the inside of the clay gives off gases that cannot escape as the clay has lost most of its porosity, so appear as bumps on the surface. Anything above 1150 (Cone 2) is pushing it with terracotta (at least those available in the UK).

If you try a Cone 5 glaze fired at, say, Cone 2, the chances are it won't have fully matured, i.e. the components won't have fully fused and melted together. So instead of a smooth, glassy surface it is more likely to be matt and possibly not smooth to touch; also, the colours may come out different. This isn't necessarily a problem, depending on what you want to achieve, but be careful if making pieces for food or drink as an underfired glaze is likely to leach out chemicals into the food - many don't do any harm at all, especially in the low quantities we are looking at, but some can be bad for your health.

Just firing once can be done, but there are a couple of differences. As the glaze is applied before the pot is fully dry, it needs to move with the pot as it shrinks, so as Babs says you may need to use a glaze with more clay in it. Secondly, the pot can give off quite a lot of gas when fired - water, CO2 from organic materials in the clay burning out etc. If the glaze melts too quickly it forms an impervious layer and the gases cannot escape easily - you may need to fire more slowly up to about 600C to allow the gases time to get out before the glaze melts.

For a matt transparent glaze, a lot of the very matt glazes are formed from micro crystals forming as the glaze cools, so what would have been a clear gloss glaze becomes a translucent white matt glaze. But other things affect the level of gloss on a glaze. For example, this is a low gloss glaze from Tony Hansen's Digital Fire that I've used successfully:

G2805 Cone 04-06 MatteFrit 3134  44Dolomite    8Kaolin     12Whiting     8Silica     28

Best thing with all of this is to try it and see - make a few simple pots (so you haven't wasted too much time if it goes wrong!), change one thing at a time to see the effect,  and see what happens. That's a more effective way of learning than just reading about things, and you are very unlikely to do any harm with what we've discussed here.

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the answers you have might have alerted you to a slight problem.  why do you want to use terra cotta?  is it because you have a lot of that particular clay?  if so, you will need to do what it wants you to do, fire it to its best temperature which is  lower than cone 5.

 

if you want to have a cone 5 pot, get some red-brown clay and use it with cone 5 glaze.  if you are in the usa, i can give you a two ingredient recipe for a lovely redbrown, clay.  it goes to cone 6 and is very smooth and easy to work with.  mix 50% redart clay with 50% XX saggar clay.  

 

deciding what you want to have as a result is usually better than deciding to do a specific thing just because you want to.  there is enough frustration in claywork without asking for trouble.  

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